1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine, and, more particularly, to a control apparatus for an internal combustion engine capable of controlling fuel injection rate and ignition timing on the basis of detected intake pressure.
2. Description of the Related Art
Conventional, internal combustion engines equipped with a control apparatus have been known. The control apparatus computes periodically a basic fuel injection period on the basis of the detected intake pressure and the detected engine speed, obtains a fuel injection period by correcting the basic fuel injection period with intake air temperature and engine cooling water temperature, and opens the fuel injection valves to inject fuel for a period of time equal to the thus-obtained fuel injection period and injects the fuel. In this internal combustion engine, an acceleration fuel increment system is employed in order to improve engine response at the time of acceleration by detecting a change rate in the detected intake pressure and correcting the basic fuel injection period by an amount which is in proportion to the thus-detected change rate.
In the above-described type of internal combustion engine which computes the basic fuel injection period on the basis of the intake pressure, a pressure sensor for sensing the intake pressure (absolute pressure) is attached to an intake pipe, and the basic fuel injection period is computed on the basis of the thus-sensed intake pressure. However, the detected values can be changed due to pulsations of the engine. These changes cause the basic fuel injection period to be changed, and correct control of, fuel inject:,on rate becomes impossible to be performed.
In view of the foregoing, as disclosed in Japanese Patent Application Laid-Open No. 59-201938, the acceleration increment is performed by using two filters which have an individual time constant for weighting the output of the pressure sensor and completely erasing the pulsation component from the output of the pressure sensor, and an overshoot characteristic is given by subtracting the filter output having a relatively large time constant from the filter output having a small time constant. Then the acceleration increment is performed in accordance with the thus-obtained difference between the filter outputs. However, in this known method in which the two filters are used, since the amount of weighting of the output from the pressure sensor is enlarged by using the filter which has a relatively large time constant for the purpose of erasing the pulsation component, the response and resulting capability of the change of output from the filter with respect to the change in the actual change of the intake pressure can deteriorate. As a result, a delay in the acceleration increment attributable to the above will cause a deficiency in the fuel injection at the transient period of the acceleration and generation of a lean spike. Furthermore, in the case of the final stage of the acceleration, a rich spike can be generated due to the overshoot characteristic.
To this end, in order to obtain a detected intake pressure of better response and following characteristics than in using the two filter, it has been recently proposed to process the output from the pressure sensor by using a CR filter which comprises a resistor and a condenser and which has a relatively reduced time constant but is capable of erasing the pulsation component, and to periodically convert the thus-obtained output from the CR filter into a digital value. In this case, since the pulsation component cannot be erased completely by the CR filter, two weighted means, each having individual relaxation or weighting amounts, are computed by using the thus-obtained digital value, that is, a digital filtering is performed, and the second weighted means having a relatively large weighting amount, is subtracted from the first weighted mean having a relatively small weighting amount so that the acceleration increment amount is determined on the basis of the thus-obtained difference.
However, since the weighted means having the large weighting amount is used to obtain the acceleration increment amount in all of the above-described known methods, the response and following characteristics deteriorate. Therefore, there arises a phase delay of the acceleration increment generated in a drive pattern in which acceleration and deceleration are repeated, causing a case that the fuel injection rate does not meet a demand from the engine to increase the fuel. Consequently, a problem arises that the emission and driveability can deteriorate. It might, therefore, be considered feasible to obtain only a small weighting value but capable of erasing the engine pulsation component from the pressure sensor output, and to compute the fuel injection rate including the acceleration increment on the basis of the thus-obtained weighting value. In this method, a certain period of time needs to be taken for the time from computing the fuel injection period to the time at which the injected fuel reaches the combustion chamber this time being attributable to the affect of computing time and the time taken for the fuel to pass through the route. What is worse, a difference is generated between the intake pressure or weighted value used at the time of computing the fuel injection period and an intake pressure corresponding to the actual intake amount. As a result, it is impossible to conduct control with the air-fuel ratio demanded by the engine secured.
This phenomenon will be described in detail with reference to FIG. 4. FIG. 4 is a view which illustrates change in the computed basic fuel injection period TP and intake pressure PM at the time of acceleration of a 4-cylinder 4-cycle internal combustion engine which has a capacity for fuel injection in the suction cycle once in one rotation of the engine by a quantity which is a half of the required quantity. In this case, since the fuel is arranged to be injected once in one rotation of the engine, that is twice in one cycle (referring to this figure, point c and point b), the quantity of fuel contributed to one combustion is, as can be clearly seen from this figure, a quantity corresponding to TPc+TPb. However, the intake pressure representing the actual amount of intake air at the time of combustion is the intake pressure illustrated by symbol a when the suction cycle is completed (at the lower dead center in the suction cycle). As described above, the existence of a time delay tD between the intake pressure at the time of computing the fuel injection period and the intake pressure representing the actual amount of intake air at the time of combustion causes is to be impossible for fuel to be injected in accordance with the actual amount of intake air. As a result, it becomes impossible to conduct control with the air-fuel ratio demanded by the engine secured. On the other hand, it might, therefore, be considered feasible to reduce the time delay tD to the extent which can be neglected by reducing the computing time or the like (if the lower dead center in the suction cycle and the point b coincide with each other). However, in the internal combustion engines which injects fuel once during one engine rotation, fuel is supplied only by a quantity, corresponding to TPc+TPb although the amount of fuel corresponding to 2TPb needs to be supplied during one cycle. As a result, the fuel quantity becomes lessened by an amount obtained by TPb-TPc (=.DELTA.TP) at the time of acceleration.
To this end, the applicant of the present invention has proposed a known method capable of correcting the amount of fuel shortage .DELTA.TP (see Japanese Patent Application No. 61-277019 (Japanese Patent Application Laid-Open No. 63-131840) and Japanese Patent Application No. 61-277020 (Japanese Patent Application Laid Open No. 63-131841).
The principle of these known arts will be described referring to a 4-cylinder 4-cycle internal combustion engine which injects fuel once during one engine rotation.
As described with reference to FIG. 4, neglecting the time delay tD after computing the fuel injection period, the basic, fuel injection period TP corresponding to the actual amount of intake air can be expressed by the following formula (1). EQU TP=TPb+.DELTA.TP (1)
On the other hand, it is assumed that the acceleration is performed at a constant speed as shown in FIG. 5. Since difference .DELTA.TP in the basic fuel injection period between that at the point b and that at the point C and the difference .DELTA.TP' in the basic fuel injection period at the point b and point b' are equal to each other, the basic fuel injection period TPb' at point b' can be expressed by the following formula (2) by using the basic fuel injection period TPb at the point b and the above-described .DELTA.TP (=TP'). EQU TPb'=TPb+.DELTA.TP (2)
Assuming that the basic fuel injection period is performed every 360.degree. CA, a basic fuel injection period advanced by 360.degree. CA from the point b is, as will be understood from the formula (2), estimated.
Accordingly, assuming that the calculation of the basic fuel injection period is performed every CY [.degree.CA], and converting the time delay tD between the point a and point b shown in FIG. 4 into a crank angle CAD, the amount of correction corresponding to this crank angle CAD can be derived as follows. ##EQU1##
As a result, the basic fuel injection period advanced by the predetermined crank angle CAD from the point b can be estimated. Therefore, considering the correction at the change from the point c to point b, basic fuel injection period TP corresponding to the actual amount of intake air when used at the time of computing the basic fuel injection period every CY [.degree.CA] can be expressed by the following formula (4) using the basic fuel injection period TP.sub.0 computed immediately before the lower dead center in the suction cycle. EQU TP=TP.sub.0 +k.multidot..DELTA.TP (4)
where k represents ##EQU2## and .DELTA.TP represents the difference obtained by subtracting the basic fuel injection period computed CY [.degree.CA] previously from the present basic fuel injection period TP.sub.0. The thus obtained difference becomes a positive value in the case of acceleration, while the same becomes a negative value in the case of deceleration.
In the case where the CR filter is used, the CR filter output can be considered to substantially represent the actual intake pressure attributable to the excellent response of the same with respect to the change in the actual change in the intake pressure. However, weighted mean (corresponding to the weighted value) for computing the basic fuel injection period is delayed, as shown in FIG. 6, behind the actual intake pressure. This delay (control delay tD') can be generated due to the delay in detection by the pressure sensor, the delay in transmitting a signal through the input circuit, the delay in computing timing due to any of the above-described types of delay, the delay in the computing period, and delay caused from weighting the CR filter outputs. Therefore, it is necessary to estimate the fuel injection period by estimating the actual intake pressure PMb taking into consideration the control delay tD' (corresponding to crank angle CAD') from the PMb' for computing the fuel injection rate at Point "b" shown in FIG. 6, computing the basic fuel injection period on the basis of the thus-obtained estimated value and consideration of the above-described time delay tD.
Therefore, including the correction of the control delay tD' (=CAD') in the above-described formula (4), the fuel injection period TP can be expressed as follows. EQU TP=TP.sub.0 +K.sub.1 .multidot..DELTA.TP (5) ##EQU3##
In a case where the basic fuel injection period TP is calculated from the intake pressure PM and engine speed NE, the formula (5) can be expressed by the following formula (6) by using the difference in the weighting value of the intake pressure (value obtained by subtracting the weighting value for computing the basic fuel injection period by CY.degree.CA earlier from the present weighting value for computing the basic fuel injection period), that is, by using the change rate .DELTA.PM in the weighting value, since TP.varies.PM EQU TP=TP.sub.0 +K.sub.1 .multidot..DELTA.PM.multidot.C (6)
where C represents a proportional constant for converting the intake pressure into the fuel injection period.
Since the above-described control time delay tD' can be assumed to be substantially constant as to the time periodical phenomenon, it is enlarged in proportion to the engine speed. The crank angle CAD' can be obtained by calculation, and the value K.sub.1 at each of the engine speeds can be obtained regardless of the error at the time of manufacturing the engines to be tested. Although the case is described in which the basic fuel injection period is computed at every predetermined crank angle (CY.degree. CA) in the above-described description, the method can be embodied in a case where the basic fuel injection period is computed periodically. In this case, although the correction of CAD' with the engine speed becomes needless, the delay is affected by the engine speed. Therefore, the overall amount of K.sub.1 needs to be subjected to correction with the engine speed. In the above description, the case where fuel is injected once during one rotation of the engine is described above. However, in the case of an individual injection system in which each of the cylinders individually injects fuel, the above described time delay tD' causes it to become impossible for fuel to be injected in accordance with the actual amount of intake air. Therefore, it is preferable to estimate the intake pressure (pressure in the vicinity of the lower dead center in the suction cycle) representing the actual amount of intake air at the time of computing the fuel injection period which is advanced by one cycle from computing the present basic fuel injection period. As a result, the method can be embodied in individual injection engines.
However, in the known method in which the basic fuel injection period TP is computed with the formulas (5) and (6), the change rate .DELTA.PM becomes too large a value at a time of rapid acceleration. This leads to the generation of an overshoot of the fuel injection period TAU as shown in FIG. 12 (1), causing the air-fuel ratio to become too rich. As a result CO and HC emissions are increased and driveability is worsened. Furthermore, in the internal combustion engines described above, since the basic ignition advance is obtained from the weighted value of the intake pressure and the engine speed, and the thus obtained basic ignition advance at the time of acceleration is corrected by the change rate .DELTA.PM, the correction of the basic ignition advance with the change rate .DELTA.PM becomes incorrect at a time of rapid acceleration. Furthermore, since the correction with the change rate .DELTA.PM becomes incorrect at the time of rapid deceleration, the fuel injection rate and ignition timing cannot meet the demand of the engine, causing worsened driveability and emission.